Metabolic dysfunction (e.g., insulin resistance, diabetes and dyslipidemia) and certain types of cancer (e.g., colon cancer) are important complications associated with obesity. Weight loss can improve or even normalize metabolic function in obese persons, and has been shown to decrease colon cancer risk by reducing cell proliferation rates in rodent models. However, weight loss decreases muscle mass and bone mineral density, which can have adverse clinical effects in susceptible subjects such as postmenopausal women because they are already at increased risk for sarcopenia, impaired physical function and osteoporosis. It has been proposed that increasing protein intake during calorie restriction (CR) could ameliorate the adverse effects of CR on the musculoskeletal system, because of the anabolic effect of protein on muscle and bone metabolism. However, it is also possible that increased protein intake will impair insulin sensitivity by activating mammalian target of rapamycin (mTOR) signaling and stimulate cell proliferation in the colon by increasing circulating growth factors. Accordingly, the overall purpose of this proposal is to perform a 9-month randomized controlled trial to determine whether an 8%-10% body weight loss induced by CR alone or CR with protein supplementation has beneficial or harmful effects on multi-organ (liver, muscle, adipose tissue) insulin sensitivity, colonocyte proliferation, muscle mass and function, and bone mineral density and bone architecture in obese, postmenopausal women. The information gained from this work will fill important gaps in our knowledge that could affect clinical practice and weight management dietary guidelines.